Anti-wear ZDDPs

Reactive FFs can only be applied to a few specific cases for which they have been developed, such as the hydrocarbon systems discussed in the first part of this section. For other systems, describing tribochemical reactions requires the use of quantum chemical methods. In recent studies, such methods have been applied to investigate the behavior of zinc phosphates (ZPs) in response to high pressures. ZPs form the basis of anti-wear films derived from zinc dialkyldithiophosphates (ZDDPs), which are additives that have... 

Overall, this work highlights how quantum chemical methods can be used to study tribochemical reactions within chemically complex lubricant systems. The results shed light on processes that are responsible for the conversion of loosely connected ZP molecules derived from anti-wear additives into stiff, highly connected anti-wear films, which is consistent with experiments. Additionally, the results explain why these films inhibit wear of hard surfaces, such as iron, yet do not protect soft surface such as aluminum. The simulations also explained a large number of other experimental observations pertaining to ZDDP anti-wear films and additives.103 Perhaps most importantly, the simulations demonstrate the importance of cross-linking within the films, which may aid in the development of new anti-wear additives. 

Lett. 24, 105 (2006). Interpretation of Experiments on ZDDP Anti-Wear Additives and Films through Pressure-Induced Cross Linking. 

ZDDP (zinc dialkyl dithiophos-phate or zinc diaryl dithiophos-phate)—widely used as an anti-wear agent in motor oils to protect heavily loaded parts, particularly the valve train mechanisms (such as the camshaft and cam followers) from excessive wear. It is also used as an anti-wear agent in hydraulic fluids and certain other products. ZDDP is also an effective oxidation inhibitor. Oils containing ZDDP should not be used in engines that employ silver alloy bearings. All car manufacturers now recommend the use of dialkyl ZDDP in motor oils for passenger car service. 

Considering these results, the main difference between the antiwear action of the ZDDP soft-core RMs and the hard-core RMs is clear. In the case of ZDDP soft-core RMs, the anti wear film formation requires that chemical reactions occur between the additive and the metallic surfaces. In the case of hard-core RMs, the mineral material (CaC03) is directly introduced to the sliding contact and undergoes small physicochemical changes during the film build-up. Consequently no chemical reaction with the substrate surfaces is required. 

MoDTC+ZDDP. A synergistic effect in reducing friction and wear has been attributed to anti-wear films containing primarily MoS2 and polyphosphates. The counter ion to the phosphates is zinc, rather than iron. The XANES spectra indicate that sulfur and phosphorus form MoS2 and polyphosphate chains (Kasrai et ah, 1998). 

Figure 4.3 Reprinted from Wear, Vol. 202, Z. Yin, M. Kasrai, M. Fuller, G.M. Bancroft, K. Fyfe and K.H. Tan, Application of soft X-ray absorption spectroscopy in chemical characterization of anti wear films generated by ZDDP the effect of physical parameters, Part I, pp. 172-191. Copyright 1997, with permission from Elsevier.

Isobutanol use in the manufacture of zinc dialkyl dithiophosphates (ZDDP), anti-wear lube oil additives, represented 13 percent of domestic consumption. Other alcohols used in this application include methylamyl alcohol, primary amyl,alcohol, n-butanol, 2-ethylhexanol and isooctanol. 

These results must be treated with caution. ZDDP s are anti-wear additives, as well as anti-oxidants and corrosion inhibitors, but are not particularly effective in increasing load-carrying capacity, so that the comparison between the two additives in this case is not a very demanding one. On the other hand, the highest concentration of molybdenum disulphide used was only 1 %, well below the concentrations normally used for load-carrying performance, which would typically be greater than 5%. 

Four-Ball Machine later to study the interaction between molybdenum disulphide and several anti-wear and extreme-pressure additives and detergent/dispersant additives in a mineral oil. Unfortunately these results are difficult to compare directly with those of Thorp because he only reported wear scar diameters at two load levels. He found that at high load (lOOON) with 1 % of molybdenum disulphide, the combination with a ZDDP gave a wear scar diameter higher than either additive separately, and comparable to that of the base oil, and he described this as an antagonistic effect between the two additives. 

Kasrai, M., Cutler, J.N., Gore, K., Canning, G.W., Bancroft, G.M. and Tan, K.H., The Chemistry of Anti-Wear Films generated by the Combination of ZDDP and MODTC Examined by X-Ray Absorption Spectroscopy, World Tribology Congress, London, 8-12 September, 1997. (STLE Preprint No. 97-WTC-9). 

As for all additives, interactions with other additives in solution, Fig. 3.16, and competition for surface reaction sites together with the effect of environmental factors such as temperature, blow-by gases, water and fuel dilution have variable effects on the formation of the film. Because ZDDPs are much more widely used as antiwear performance additives compared to other classes of compounds, these additive effects will now be discussed in greater detail than has been the case for other classes of anti-wear/friction additives. In particular the influence of structure, concentration, dispersant, detergent, antioxidancy and friction modifier on friction and wear will be discussed. In addition the influence of NO c and H2O will be briefly illustrated. 

It follows from this that the ZDDPs which are most efficient at anti-wear film formation will also be likely to suffer depletion due to thermal effects. The thermal degradation of ZDDPs in service has often been confirmed by P NMR and IR studies [45] and is not really important unless decomposition proceeds to the stage where significant reduction in phosphorus and sulphur levels as insoluble or volatile products occurs. Indeed, according to [6], the early stage decomposition products are ... 

Figure 3.17 shows how high temperature affects the anti-wear protection of a passenger car engine oil formulation with a secondary ZDDP and with one based on primary ZDDP. The superior initial wear protection of the secondary ZDDP is destroyed much more quickly than that of the primary ZDDP oil. Blends of the two ZDDPs gave good initial wear protection and good sustained wear protection. 

Of this latter group, the most effective inhibitors of high-temperature oxidation are claimed to be the aromatic amines [56], but these are capable of complexing ZDDPs and hence inhibiting their anti-wear performance. Careful balance of ZDDP and antioxidant(s) is required to achieve both wear and viscosity increase control. 

Both low friction and low wear can be achieved if these films of friction modifier are laid down on thicker films of, say, ZDDP. The anti-wear and friction properties of several classes of additives have been described together with the influence of... 

Structure. In the case of ZDDP, the role of other additives on film formation and antiwear behaviour has been discussed. Successful control of anti-wear and anti-friction properties of oils requires a care fill balance of the additives in the formulation. 

The principal problem, which is compounded by the above, is that the most cost-effective source of anti-wear film formation and the most effective peroxide decomposing agent has, since the 1940s, been ZDDPs which contain both sulphur and phosphorus as the active elements. 

The Ui method permits a better understanding of the interactions observed between ZDDP (zinc dialkyldithiophosphates) additives and various mineral base oils [17]. This applies for the anti-wear (AW), extreme pressure (EP) and antioxidant functions of ZDDP. The viscosity of base oils was shown to be a factor controlling these relationships. The o method has also been found useful in showing the directions of the relationships. For the case when an observed change should be considered as a complex one, the influence of the tested medium on it may be expressed as a sum as follows ... 

Additive Loss in Lubricants Additive loss is detrimental to the performance of lubricants. Some additive levels can be monitored by infrared spectroscopy. For instance, anti-wear additives, zinc dithio-dialkyl (diaryl) phosphate (ZDDP) and tri-cresyl phosphate (TCP) contain a common phosphate functional group that can be measured by infrared. The P-O-R (where R = alkyl/aryl) stretch shows a strong IR absorbance for all of these compounds and is used to trend the anti-wear level. The P-O-R stretch area is measured over the region of 1020-960 cm using the general baseline of 2000-600 cm. ... 

One area of question that was not part of this study but should be considered is the impact of phosphorus volatilization on the residual oil from the special Noack test. That is, to what extent is the anti-wear and anti-oxidation performance of a ZDDP compromised by phosphorus volatilization This information is obviously of high interest and importance given the primary purpose of ZDDPs in engine oils. 

Zinc Di(organo) Di(thio) Phosphates, commonly referred to as ZDDPs, are the most widely used and effective anti-wear/anti-oxidation additives in engine oil. Phosphorus contained in the ZDDP molecules has been shown to partially volatize during engine operation [2]. Unfortunately, volatile phosphorus in the exhaust stream degrades the iunction of the exhaust catalytic converter, and as a consequence, there has been pressure to reduce the amount of ZDDP in engine oil. 

The volatile components from ZDDPs may also arise from thermal degradation and/or oxidation. The mechanisms proposed for ZDDP anti-wear and anti-oxidation performance in motor oil have included both the effects of ZDDP degradation and oxidation [2], In addition, P NMR studies at other laboratories have identified ZDDP breakdown fragments in the used motor oil [12], as shown in... 

Complementary analytical studies of the surface films using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edges structures (XANES) [7, 8] lead to an accurate description of the antiwear film mainly composed of amorphous Ee/Zn poly(thio)phosphate embedding some nanocrystallites of ZnO and ZnS. The chemical reaction involved in the build-up of the tribo-film were recently interpreted with new theoretical approaches (hard and soft acid-base theory) [9, 10]. The action process of anti wear additives such as ZDDP presents two disadvantages ... 

Figure 4.2 (a) TEM image recorded onto an anti-wear film particle and corresponding electron diffraction pattern, (b) X-ray emission spectrum (EDXS) analysis. The antiwear film is composed of iron and elements present in the ZDDP additive (O, P, S, Zn). (c) and (d) Fe and Zn radial distribution functions (RDFs) uncorrected from phase shift, extracted from EXAFS experiments on ZDDP antiwear film particles. The electron diffraction pattern and the presence of one major peak in each RDF corresponding to oxygen first-neighbours shell point out the amorphous structure of the antiwear film, (e) Schematic structure of the ZDDP tribo-film according to the multitechnique approaches [8]... 

In this paper the effects of eliminating the friction modifier and friction modifier plus anti-wear additive ZDDP from the additive package of fully formulated lubricants on friction, wear and wear film forming characteristics are examined. Tests have been conducted under lubricated wear conditions at relatively low (20°C and 50°C) and elevated (up to 100°C) bulk oil temperatures using a reciprocating pin-on-plate tribometer. The wear film has been examined by Energy Dispersive X-Ray analysis (EDX) and X-Ray Photoelectron Spectroscopy (XPS). 

The anti-wear performance of the additive packages is often insured by ZDDP additive. How ZDDP additives form protective films on rubbing surfaces has been a concern for researchers in the field of tribology since the performance of the additive was first reported. The mechanism of wear film formation is important for two reasons - firstly to understand how current additives function and hence determine the limits of their usefulness and... 

For the performance of tribological contacts the important parameters in terms of achieving fuel economy are friction and wear. ZDDP is most commonly used as an anti wear additive plus it often has anti-oxidation properties [1]. The effect of ZDDP on friction performance is often complicated. In the literature, there have been reports where ZDDP formulated lubricants produced higher friction than lubricant formulations without ZDDP [12-16]. Holinski [12] used the Bartel lubrimeter which was designed to test gear oils under boundary conditions. He investigated the effect of boundary layers formed from different additives on friction and wear, and found that when the lubricant is... 

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